Volume 04 - Heat Treating Part 4 ppsx

• Workpiece characteristics (part geometry and electrical-magnetic properties) • Coupling distance and coil design • Frequency selection • Power density (kilowatts per unit area exposed to the inductor) • Heating time The choice of suitable heating parameters is, to a large extent, determined by the temperature required and the depth of heating. Workpiece characteristics are also important. The distribution of induced current is influenced also by the magnetic and electrical characteristics of the part being heated, and because these properties change with temperature (see discussion above), the current distribution will change as the work is heated. Optimum heating for a given workpiece and heat treatment requires detailed knowledge of the application and equipment. Initial guidance can come from charts or calculations for a specific set of conditions. Many induction heating equipment manufacturers have extensive computer programs based upon laboratory tests and production/operating data, which they use to recommend the proper apparatus and suggest application parameters. An estimate of what may be required for a new application can often be derived from results obtained on similar parts or by careful observation of the part itself as it is being heat treated. Final operating parameters are usually determined by experimentation. Basic process control for most induction heating applications consists of applying power through a voltage-regulated power supply, for a measured period of time, and this has proved to be satisfactory for a wide variety of operations. Solid state inverters through their logic circuits can provide constant voltage, constant current, or constant power output and each in a particular way can help to ensure a repeatable heating effect with time under a wide variety of changing conditions. For a stationary hardening operation either an electronic or a synchronous timer can be used to control the heating time, any needed load-matching adjustments, and application of the quench. If energy input to the product is considered an appropriate measure of control, a kilowatt-second or kilowatt-hour energy monitor can be used to terminate a heating cycle. Typical heating and energy requirements for various induction processes are listed in Tables 5 and 6. Table 5 Approximate induction heating temperatures required for typical metalworking processes Required temperature, °C (°F), for processing of: Stainless steel Process Carbon steel Magnetic Nonmagnetic Nickel Titanium Copper Brass Aluminum Hot forging 1230 (2250) 1095 (2000) 1150 (2100) 1095 (2000) 955 (1750) 900 (1650) 815 (1500) 540 (1000) Hardening 925 (1700) 980 (1800) . . . 760 (1400) 900 (1650) 815 (1500) 650 (1200) 480 (900) Annealing/normalizing 870 (1600) 815 (1500) 1040 (1900) 925 (1700) 815 (1500) 540 (1000) 540 (1000) 370 (700) Warm forging 760 (1400) . . . 650 (1200) 650 (1200) . . . . . . . . . . . . Stress relieving 595 (1100) 595 (1100) 595 (1100) 595 (1100) 595 (1100) 280 (500) 290 (550) 370 (700) Tempering 315 (600) 315 (600) 315 (600) 315 (600) 315 (600) . . . . . . . . . Curing of coatings 230 (450) 230 (450) 230 (450) 230 (450) 230 (450) 230 (450) 230 (450) 230 (450) Table 6 Average energy requirements for induction heating in typical metalworking processes Required energy (a) , kW · h/ton, for processing of: Stainless steel Process Carbon steel Magnetic Nonmagnetic Nickel Titanium Copper Brass Aluminum Hot forging 400 375 430 450 375 700 400 300 Hardening/aging 250 260 . . . 300 325 600 325 275 Annealing/normalizing 225 210 375 400 300 425 375 210 Warm forming 175 . . . 250 240 . . . . . . . . . . . . Stress relieving 150 150 200 250 225 200 200 210 Tempering 70 70 100 120 110 . . . . . . . . . (a) Based on in-line continuous process Frequency Selection Frequency is the first parameter considered for induction heating. Primary considerations in the selection of frequency are depth of heating, efficiency, type of heat treatment (such as surface hardening versus subcritical annealing), and the size and geometry of the part. The frequencies and power supplies commonly used in the induction hardening of steel are compared in Table 7. As shown in this tabulation, the lower frequencies are more suitable as the size of the part and the case depth increase. However, because power density and heating time also have an important influence on the depth to which the part is heated, wide deviations from Table 7 may be made with successful results. This interrelationship is shown in Fig. 39 in terms of case depth for surface hardened steel. In some instances, the determining factor in selecting the frequency is the power required to provide power density sufficient for successful hardening, as lower-frequency induction equipment is available with higher power ratings. Table 7 Selection of power source and frequency for various applications of induction hardening and tempering of steel Heat-treatment criterion Section size Power lines, Frequency converter, Solid state or motor Vacuum tube, generator mm. in. 50 or 60 Hz 180 Hz 1000 Hz 3000 Hz 10,000 Hz over 200 kHz Surface hardening depth Surface hardening ratings (a) 0.38-1.27 mm 0.015-0.050 in. 6.35-25.4 1 4 -1 . . . . . . . . . . . . . . . Good 11.11- 15.88 7 16 - 5 8 . . . . . . . . . . . . Fair Good 15.88- 25.4 5 8 -1 . . . . . . . . . . . . Good Good 25.4-50.8 1-2 . . . . . . . . . Fair Good Fair 1.29-2.54 mm 0.051-0.100 in. >50.8 >2 . . . . . . Fair Good Good Poor 19.05- 50.8 3 4 -2 . . . . . . . . . Good Good Poor 50.8- 101.6 2-4 . . . . . . Good Good Fair . . . 2.56-5.08 mm 0.101-0.200 in. >101.6 >4 . . . . . . Good Fair Poor . . . Through hardening Through hardening ratings (b) 1.59-6.35 1 16 - 1 4 . . . . . . . . . . . . . . . Good 6.35-12.7 1 4 - 1 2 . . . . . . . . . . . . Fair Good Through hardening based on heating rate of carbon steel in Fig. 40 (b) 12.7-25.4 1 2 -1 . . . . . . . . . Fair Good Fair 25.4-50.8 1-2 . . . . . . Fair Good Fair . . . 50.8-76.2 2-3 . . . . . . Good Good Poor . . . 76.2- 152.4 3-6 Fair Good Good Poor Poor . . . >152.4 >6 Good Fair Poor Poor Poor . . . Maximum tempering temperature Tempering ratings (c) 705 °C 1300 °F 0.32-0.64 1 8 - 1 4 . . . . . . . . . . . . . . . Good 705 °C 1300 °F 0.64-1.27 1 4 - 1 2 . . . . . . . . . . . . Good Good 425 °C 800 °F 1.27-2.54 1 2 -1 . . . Fair Good Good Good Fair 705 °C 1300 °F . . . Poor Fair Good Good Fair 425 °C 800 °F 2.54-5.08 1-2 Fair Fair Good Good Fair Poor 705 °C 1300 °F . . . Fair Good Good Fair Poor 425 °C 800 °C 5.08- 15.24 2-6 Good Good Good Fair . . . . . . 705 °C 1300 °F Good Good Good Fair . . . . . . 705 °C 1300 °F >15.24 >6 Good Good Good Fair . . . . . . (a) Surface hardening ratings: Good indicates frequency that will most efficiently heat the material to austenitizing temperature for the specified depth. Fair indicates a frequency that is lower than optimum but high enough to heat the material to austenitizing temperature for the specified depth. With this frequency, the current penetration relative to the section size causes current cancellation and lowered efficiency. Poor indicates a frequency that will overheat the surface unless low-energy input is used. Efficiency and production are low, and capital cost of converters per kilowatt-hour is high. (b) Through hardening ratings: Good based on heating rates in Fig. 40(b). Fair is based on a smaller heating rate, but fair may also indicate a frequency higher than optimum that can overheat the surface at high-energy inputs. Converters cost more per kilowatt-hour than the converters of optimum frequency. With some equipment, the efficiency may be lower. Poor indicates a frequency that will overheat the surface unless low-energy input is used. Efficiency and production are low and capital cost of converters per kilowatt-hour is high. (c) Tempering ratings are based on efficiency, capital cost, and uniformity of heating. Good indicates optimum frequency. Fair indicates a frequency higher than optimum that increases capital cost and reduces uniformity of heating, thus requiring lower heat inputs. Poor indicates a frequency substantially higher than optimum that substantially increases capital cost and reduces uniformity of heating, thus requiring substantially lower heat inputs. Fig. 39 Interrelationship among heating time, surface power density, and hardened depth for various induction generator frequencies The equation given earlier in this article for reference depth, d, can be used to estimate the optimal generator frequency for induction hardening of steel . For surface hardening, the desired case depth is typically taken to be equal to about one- half the reference depth when selecting frequency. By contrast, when through-hardening is desired, the frequency is usually chosen such that the reference depth is a fraction of the bar radius (or an equivalent dimension for parts which are not round). This is necessary in order to maintain adequate "skin effect" and to enable induction to take place at all. If the reference depth is chosen to be comparable to or larger than the bar radius, there will be two sets of eddy currents near the center of the bar induced from diametrically opposed surfaces of the bar. These will tend to go in two different directions and thus cancel each other. To avoid this, frequencies for through-hardening are often chosen so that the reference depth does not exceed approximately one-fourth of the diameter for round parts or one-half the thickness for plates and slabs when using solenoid coils. When the bar diameter is less than four reference depths, or slab thickness less than two reference depths, the electrical efficiency drops sharply. By contrast, little increase in efficiency is obtained when the bar diameter or slab thickness is many times more than the reference depth. Typical frequency selections for induction hardening of steel parts are listed in Table 7 and Fig. 40. Those for surface hardening will be examined first. For very thin cases such as 0.40 to 1.25 mm (0.015 to 0.050 in.) on small-diameter bars, which are easily quenched to martensite, relatively high frequencies are optimal. If the reference depth is equated to the case depth, the best frequency for a 0.75 mm (0.030 in.) deep case on a 13 mm (0.5 in.) diameter bar is found to be around 550 kHz. When the surface of a larger-diameter bar is hardened, particularly when the case is to be deep, the frequency is often chosen so that the reference depth is several times the desired case depth. This is because the large amount of metal below the surface layer to be hardened represents a large thermal mass which draws heat from the surface. Unless very high power densities are employed, it is difficult to heat only the required depth totally to the austenitizing temperature. As an example, consider the recommended frequency for imparting a 3.8 mm (0.15 in.) hardened case to a bar 75 mm (3 in.) in diameter. If the reference depth were equated to the case depth, a frequency of about 20 kHz would be selected, which would provide only "fair" results. If a frequency of 3 kHz were chosen, however, the reference depth would be about 10 mm (0.41 in.), or about 2 1 2 times the required case depth. However, it is unlikely that the entire reference depth would ever reach austenitizing temperatures for the reason mentioned above. Fig. 40 Typical freque ncy selections and heating rates for induction hardening of steel parts. (a) Relationship between diameter of round steel bars and minimum generator frequency for efficient austenitizing using induction heating. (b) Heating rate for through heating of carb on steels by induction. For converted frequencies, the total power transmitted by the inductor to the work is less than the power input to the machine because of converter losses. See also Table 7. For through-hardening of a steel bar or section, the optimal frequency is often based on producing a reference depth about one-fourth of the bar diameter or section size. For instance, through-heating and through-hardening of a 64 mm (2.5 in.) diameter bar would entail using a generator with a frequency of about 1 kHz. If much lower frequencies were employed, inadequate skin effect (current cancellation) and lower efficiency would result. On the other hand, higher frequencies might be used. In these cases, however, the generator power output would have to be low enough to allow conduction of heat from outer regions of the steel part to the inner ones. Otherwise, the surface may be overheated, leading to possible austenite grain growth or even melting. Power Density and Heating Time Once the frequency has been selected, a wide range of temperature profiles can be produced by varying the power density and heating time. Selection of these two heating parameters depends on the inherent heat losses of the workpiece (from either radiation or convection losses) and the desired heat conduction patterns of a particular application. In through-heating applications, the power needed is generally based on the amount of material that is processed per unit time, the peak temperature, and the material`s heat capacity at this temperature. Power specification for other operations, such as surface hardening of steel, is not as simple because of the effects of starting material condition and the desired case depth. Surface heating is used primarily in the surface hardening of steel parts such as shafts and gears. In this type of application, high power densities and short heating times are used when thin case depths are desired. Typical power ratings for surface hardening of steel are given in Table 8. These are based on the need to heat to austenitizing temperature (Table 4) very rapidly and have proven to be appropriate through the years of experience. When using these or other fixed ratings, however, the effect of heating time on case depth (Fig. 39) must be considered. Table 8 Power densities required for surface hardening of steel Input (b)(c) Depth of hardening (a) Low (d) Optimum (e) High (f) Frequency, kHz mm in. kW/cm 2 kW/in. 2 kW/cm 2 kW/in. 2 kW/cm 2 kW/in. 2 0.381-1.143 0.015-0.045 1.08 7 1.55 10 1.86 12 500 1.143-2.286 0.045-0.090 0.46 3 0.78 5 1.24 8 1.524-2.286 0.060-0.090 1.24 8 1.55 10 2.48 16 2.286-3.048 0.090-0.120 0.78 5 1.55 10 2.33 15 10 3.048-4.064 0.120-0.160 0.78 5 1.55 10 2.17 14 2.286-3.048 0.090-0.120 1.55 10 2.33 15 2.64 17 3 3.048-4.064 0.120-0.160 0.78 5 2.17 14 2.48 16 4.064-5.080 0.160-0.200 0.78 5 1.55 10 2.17 14 5.080-7.112 0.200-0.280 0.78 5 1.55 10 1.86 12 1 7.112-8.890 0.280-0.350 0.78 5 1.55 10 1.86 12 (a) For greater depths of hardening, lower kilowatt inputs are used. (b) These values arc based on use of proper frequency and normal overall operating efficiency of equipment. These values may be used for both static and progressive methods of heating; however, for some applications, higher inputs can be used for progressive hardening. (c) Kilowattage is read as maximum during heat cycle. (d) Low kilowatt input may be used when generator capacity is limited. These kilowatt values may be used to calculate largest part hardened (single-shot method) with a given generator. (e) For best metallurgical results. (f) For higher production when generator capacity is available Through Heating. Power ratings for through hardening of steel are much lower than those for surface hardening to allow time for the heat to be conducted to the center of the workpiece. After awhile, the rates of increase of the surface and center temperatures become comparable due to conduction, and a fixed temperature differential persists during further heating. Using methods described by Tudbury (see the Selected References at the end of this article), the allowable temperature differential permits the generator power ratings to be selected. The basic steps in selecting the power rating are as follows: • Select the frequency and calculate the ratio of bar diameter (or section size) to reference depth, a/d . For most through-heating applications, this ratio will vary from around four to six • Using the values of the thermal conductivity (in W/in. · °F) and a/d , estimate the induction thermal factor, K T (Fig. 41) • The power per unit length is calculated as the product of K T and the allowable temperature differential (in °F) between the surface and center, T s - T c . Multiplying this by the length of the bar yields the net power required in kilowatts In addition to these estimates, radiation heat loss must also be considered when determining power ratings. The upper limit of radiation losses, which is defined by the emission characteristics of a blackbody, is shown in Fig. 42 as a function of temperature. Actual workpiece materials will exhibit less radiation loss than in Fig. 36 because they do not have the broad spectral range of blackbodies. Fig. 41 Induction thermal fact or for round bars as a function of the ratio of bar diameter to reference depth (a/d) and the thermal conductivity Fig. 42 Ra diation heat loss as a function of surface temperature. Losses are based on blackbody radiation into surroundings at 20 °C (70 °F). In order to avoid calculations of power requirements, tables of power densities ordinarily used for through heating of steel (for hardening as well as other uses, such as forging) are available. One such listing is shown in Table 9. These values of power densities are based on typical electrical efficiencies and proper selection of frequency (which lead to a/d ratios in the range of four to six). It may be noted that the larger-diameter bars, which can be heated efficiently with lower-cost, lower-frequency power supplies, typically employ smaller power densities than small-diameter bars (see Table 10). This is because of the greater times required for heat to be conducted to the center of the larger pieces. Also, it can be seen that lower frequencies such as 60 and 180 Hz are not ordinarily recommended for through heating of steel when temperatures above approximately 760 °C (1400 °F) are desired. This is due to the increased reference depth (and decreased skin effect) above the Curie temperature where the relative magnetic permeability drops to unity. An exception to this practice is the use of 60 Hz sources for induction heating of very large parts such as steel slabs in steel mills. Tempering treatments may also use 60 Hz sources (Table 11). Table 9 Approximate power densities required for through-heating of steel for hardening, tempering, or forming operations Input (b) 150-425 °C (300-800 °F) 425-760 °C (800-1400 °F) 760-980 °C (1400-1800 °F) 980-1095 °C (1800-2000 °F) 1095-1205 °C (2000-2200 °F) Frequency (a) , Hz kW/cm 2 kW/in. 2 kW/cm 2 kW/in. 2 kW/cm 2 kW/in. 2 kW/cm 2 kW/in. 2 kW/cm 2 kW/in. 2 60 0.009 0.06 0.023 0.15 (c) (c) (c) (c) (c) (c) [...]... 0.053 0. 34 1 15 16 14B35H 180 24 196 2.76 7.0 50 120 565 1050 195 42 9 0.031 0.20 Flats 16 5 8 1038 60 88 123 0.59 1.5 40 100 290 550 144 9 31 94 0.0 14 0.089 19 3 4 1038 60 100 1 64 0.79 2.0 40 100 315 600 1576 347 4 0.013 0.081 22 7 8 1 043 60 98 312 1.50 3.8 40 100 290 550 1609 3 548 0.008 0.050 25 1 1 043 60 85 2 54 1.22 3.1 40 100 290 550 1365 3009 0.011 0.068 1 043 60 90 328 1.57 4. 0 40 100 290 550 148 3 3269... 195 42 9 0 . 048 0.31 Flats 16 5 8 1038 3000 300 11.3 0.59 1.5 20 70 870 1600 144 9 31 94 0.361 2.33 19 3 4 1038 3000 332 15 0.79 2.0 20 70 870 1600 1576 347 4 0.319 2.06 22 7 8 1 043 3000 336 28.5 1.50 3.8 20 70 870 1600 1609 3 548 0.206 1.33 25 1 1036 3000 3 04 26.3 1.38 3.5 20 70 870 1600 1595 3517 0.225 1 .45 1036 3000 344 36.0 1.89 4. 8 20 70 870 1600 1678 3701 0.208 1. 34 1037 mod 3000 580 2 54 0. 94 2 .4 20... varied and include both single-shot and scanning techniques Steel Surface hardness, HRC Method of hardening 41 40 3 6 -4 2 Through-hardened 43 20 4 0 -4 6 Carburized to 1. 0-1 .3 mm (0 . 040 -0 .050 in.) 1137 4 2 -4 8 Induction hardened 3.0 mm (0.120 in.) min effective depth and 40 HRC Fig 46 Comparison of fatigue life of induction surface hardened transmission shafts with that of through-hardened and carburized shafts... kW Total heating time, s Scan time Work temperature Entering coil mm Production rate Inductor input(b) Leaving coil s/cm in s/in °C °F °C °F kg/h lb/h kW/cm2 kW/in.2 Rounds 13 1 2 41 30 9600 11 17 0.39 1 50 120 565 1050 92 202 0.0 64 0 .41 19 3 4 1035 mod 9600 12.7 30.6 0.71 1.8 50 120 510 950 113 250 0.050 0.32 25 1 1 041 9600 18.7 44 .2 1.02 2.6 50 120 565 1050 141 311 0.0 54 0.35 29 49 1 1 8 1 041 9600... through-hardening of steel parts by induction Section size Material Frequency(a), Hz Power(b), kW Total heating time, s Scan time Production rate Work temperature Entering coil mm Inductor input(c) Leaving coil s/cm in s/in °C °F °C °F kg/h lb/h kW/cm2 kW/in.2 Rounds 13 49 1 1 1 041 1 8 15 16 1 041 14B35H 0.39 1 75 165 510 950 92 202 0.067 0 .43 21 17 0.39 1 510 950 925 1700 92 202 0.122 0.79 180 28.5 68 .4. .. 0.011 0.068 1 043 60 90 328 1.57 4. 0 40 100 290 550 148 3 3269 0.009 0.060 29 1 1 8 Irregular shapes 17.533 11 16 5 1 16 1037 mod 9600 192 64. 8 0. 94 2 .4 65 150 550 1020 2211 48 75 0 . 043 0.28 17.529 11 16 1 1 8 1037 mod 9600 1 54 46 0.67 1.7 65 150 42 5 800 2276 5019 0 . 040 0.26 (a) Power transmitted by the inductor at the operating frequency indicated For converted frequencies, this power is approximately... 250 0.062 0 .40 20.6 28.8 0.71 1.8 620 1150 955 1750 113 250 0.085 0.55 180 33 98.8 1.02 2.6 70 160 620 1150 141 311 0.0 54 0.35 19.5 44 .2 1.02 2.6 620 1150 955 1750 141 311 0.057 0.37 180 36 1 14 1.18 3.0 75 165 620 1150 153 338 0.053 0. 34 9600 29 1 1035 mod 38 9600 25 3 4 20 9600 19 41 30 180 9600 1 2 19.1 51 1.18 3.0 620 1150 955 1750 153 338 0.050 0.32 180 35 260 2.76 7.0 75 165 635 1175 195 42 9 0.029... increasingly automated process Often, parts are induction hardened and tempered in-line One such line for heat treating of automotive parts is depicted schematically in Fig 47 It includes an automatic handling system, programmable controls, and fiber-optic sensors Mechanically, parts are handled by a quadruple-head, skewed-drive roller system (QHD) after being delivered to the heattreatment area by a conveyor... both of which must be tailored to the particular part to be heat treated and the temperature at which the heat treatment is to be carried out In the automotive and oil-drilling equipment industries, production rates are high and the induction heat- treating method finds wide application In situations where only a few parts of a given design are to be made, induction heat treatment is usually not economically... carburized and slow-cooled parts are often reheated in selected areas by induction heating Some typical induction surface hardened steels are: • • • Medium-carbon steels, such as 1030 and 1 045 , used for automotive drive shafts, gears, and so forth High-carbon steels, such as 1070, used for drill and rock bits, hand tools, and so forth Alloy steels used for bearings, automotive valves, and machine-tool components . 3 . 048 -4 .0 64 0.12 0-0 .160 0.78 5 1.55 10 2.17 14 2.28 6-3 . 048 0.09 0-0 .120 1.55 10 2.33 15 2. 64 17 3 3 . 048 -4 .0 64 0.12 0-0 .160 0.78 5 2.17 14 2 .48 16 4. 06 4- 5 .080 0.16 0-0 .200. 0.38 1-1 . 143 0.01 5-0 . 045 1.08 7 1.55 10 1.86 12 500 1. 14 3-2 .286 0 . 045 -0 .090 0 .46 3 0.78 5 1. 24 8 1.52 4- 2 .286 0.06 0-0 .090 1. 24 8 1.55 10 2 .48 16 2.28 6-3 . 048 0.09 0-0 .120. forming operations Input (b) 15 0 -4 25 °C (30 0-8 00 °F) 42 5-7 60 °C (80 0-1 40 0 °F) 76 0-9 80 °C ( 140 0-1 800 °F) 98 0-1 095 °C (180 0-2 000 °F) 109 5-1 205 °C (200 0-2 200 °F) Frequency (a) , Hz kW/cm 2